In most cases, static pressure bearings are used for the main shaft of a large-caliber astronomical telescope (especially azimuth shaft), hydraulic oil is injected between a pair of smooth surfaces, to form a pressure oil membrane to support the load, and also to lubricate the bearing surfaces. It has the advantages that the bearing lubricating surfaces feature high rigidity, long service life and low start power, able to carry very high load with small dynamic and static friction variations. This whole system is referred to as an oil pad system. The pressure of the oil membrane is produced by the hydraulic pump. The oil flow varies cyclically during the suction and compression cycles of the hydraulic pump, and this flow pulsation produces a pulsation in pressure when it encounters impedance in the system. Finally, it results in hydraulic vibration and it spreads to the whole telescope system via the outlet.
In the extra-low speed operation of the main shaft system of the large astronomical telescope, this non-linear interference of the oil pad vibration produces a fairly obvious impact, resulting in unstable tracking speed and reduced tracking precision. Normally mechanical means is used as a solution in the design, erection and commissioning the oil pad system: (A) reducing the oil trap in the plunge pump, using gear with as many teeth and as low modulus as possible in the gear pump, and reasonably designing the unloading slot shape and dimensions in the pump covers on both sides to minimize oil trap, to reduce pressure fluctuation. (B) Preventing air pocket in the pump by using a suction pipe with a bigger diameter to reduce local clogging in the piping. (C) Avoiding bending and deformation of hydraulic cylinder piston rod or over-tightening oil seal, to avoid noise due to journal blocking in the movement. However, the adjusted oil pad usually still produces some vibration, which in some cases directly results in failure of the telescope tracking precision to meet the specification. In this case, two methods are usually adopted because the cost is quite high to re-design or transform the oil pad: (1) increasing the system rigidity with the telescope control system to suppress the disturbance, and the normal practice includes increasing the control gain for the position ring or speed ring. However, this will increase the closed loop band width, making it more easily affected by interference, and also affecting the system stability. (2) Using a pulsing attenuator, or hydraulic filter. Commonly used hydraulic filters include resonance type, cavity type, resistive type and compound type. A resonance type filter features good filtering characteristics at and close to its resonance point, and is suitable to pressure pulsation with constant frequency, therefore the filtering band width is narrow; the cavity filter is a low-pass filter, suitable in eliminating pulsation over the medium frequency, but not suitable to low frequency, as its structure will be excessively big when the filtering frequency is lowered; the resistive filter does not work to either flow pulsation or pressure pulsation, with big pressure loss, therefore its application is limited; a compound filter can produce a better filtering effect than a single one, but the pressure loss is high through multiple stages, and a big structure is required. The disadvantages of using a pulsation attenuation device is: (a) hydraulic filters are expensive. (b) Such a filter can effectively reduce pulsation if the hydraulic pump speed is constant or varies in a small range. If the hydraulic pump speed varies over a large range, the oil pad vibration band will shift, this filter cannot work and another filter for the suitable bank must be used instead.
The impact from the hydraulic system to the telescope is transmitted to the telescope position feedback scale, so its position, speed and acceleration parameters include the oil pad vibration component, seriously affecting the precision of the telescope when tracking celestial bodies.